Controller for a multiphase motor

Abstract: A positioning table device is disclosed, which includes a position detecting system for detecting the position of a table, a multiphase motor having a plurality of coils, for moving the table, a designating system for designating magnitude of an energizing electric current for each coil, a selecting system for selecting one or more of the coils to be energized, in accordance with the position of the table, a servo circuit for controlling an electric current energizing the selected coil or coils so that it becomes equal to the level of the designated magnitude, and a controller for slowing a change in an energizing electric current magnitude signal to be applied to the servo circuit upon coil selection and change so as to substantially avoid a change in magnitude of combined electric current to each coil.

Description: This invention relates to a multiphase motor arrangement usable in a precise machining instrument or a semiconductor device manufacturing exposure apparatus, for example. More particularly, the invention is concerned with a multiphase motor arrangement having a position detector for detecting the position of a table and a multiphase type motor for selectively changing coils to be energized, in accordance with the position of the table. Figure 6 is a schematic view of a positioning table device. As illustrated, the device comprises linear motor coil means 104 having four coils (four phases) arrayed along a straight line. The linear motor coil means 104 is supported by a coil supporting member 103, and a pair of guides 102 extends in parallel to the array of the coils constituting the linear motor coil means 104. Mounted on the guides 102 are a pair of upper and lower table top plates 101 which are disposed to sandwich the linear motor coil means 104 therebetween, with the intervention of guide means 112. These top plates 101 can be slidably moved in the direction of the coil array while being guided by the guides 102. The guide means 112 comprises any one of rolling, sliding and static pressure guiding means, for example, which can be selected suitably in consideration of the positioning precision or easiness of maintenance, for example. Each of the upper and lower top plates 101 is provided with movable magnet means 105 which comprises four permanent magnets disposed along the coil array with their magnetic poles disposed alternately. These movable magnets 105 are mounted to the table top plate 101 through the linear motor coil means 104 so that different magnetic poles are opposed to each other. Provided at the side of the guide 102 is a position detector 106 such as a linear scale, for example, for detecting the position of the table top plate 101. Connected to this position detector 106 is a counter unit 107 for detecting a position signal of the table. Connected to this counter unit 107 is a phase changing controller 109 for changing the coil phase. The controller 109 serves to actuate switches 110 in accordance with the table position signal from the counter unit 107. The coil phase is connected to corresponding switch 110 through a corresponding current amplifier 111, such that in response to coil selection signals outputted from the controller 109 the switches 110 are actuated to change the voltage application to the coils. Connected to the counter unit 107 is a servo controller 108 which serves to calculate a difference between the target position of the table and the current position thereof as detected through the counter unit 107, and to supply corresponding signals to the switches 110. Figure 7 shows an example of linear motor coil energization pattern in a positioning table device. Here, for convenience of explanation, four coils constituting linear motor coil means are denoted in the drawing as coil 1, coil 2, coil 3 and coil 4, in an order from left to right. Reference characters A and B each denotes the direction of flow of electric current, flowing through the coil. That is, character A denotes the flow of current in the direction of depth (forward) as viewed in the sheet of the drawing, and character B denotes the flow of current in the backward direction as viewed in the sheet of the drawing. It is seen from portions (a) - (h) of Figure 7 that the coil to be energized as well as the direction of electric current flowing through the coil are changed in accordance with the position of the movable magnet 105, to produce a thrust in the same direction by which the movable magnet 105 is moved in a predetermined direction and to cause movement of the table. Figure 12 is a schematic view of another example of positioning table device. As illustrated, the device includes linear motor coil means 104 having four coils disposed along a straight line. The linear motor coil means 104 is supported by a coil supporting member 103, and a pair of guides 102 extend in parallel to the array of the coils constituting the linear motor coil means 104. Mounted on the guides 102 through guide means 112 is a table top plate 101 which can be slidably moved in the direction of the coil array while being guided by the guides 102. The guide means 112 comprises any one of rolling, sliding and static pressure guiding means, for example, which can be selected suitably in consideration of the positioning precision or easiness of maintenance, for example. The table top plate 101 is provided with movable magnet means 105 which comprises four permanent magnets disposed along the coil array with their magnetic poles disposed alternately. Provided at the side of the guide 102 is a position detector 106 such as a linear scale, for example, for detecting the position of the table top plate 101. Connected to this position detector 106 is a counter unit 107 for detecting a position signal of the table. Connected to this counter unit 107 is a phase changing controller 109 for changing the coil phase. The controller 109 serves to actuate switches 110 in accordance with the table position signal from the counter unit 107. The coil phase is connected to corresponding switch 110 through a corresponding current amplifier 111, such that in response to coil selection signals outputted from the controller 109 the switches 110 are actuated to energize the coils. Connected to the counter unit 107 is a servo controller 108 which serves to calculate (e.g. by PID calculation) a difference between the target position of the table and the current position thereof as detected through the counter unit 107, and to supply corresponding signals to a multiplier 130. It is known that, in a linear motor structure of such arrangement as described, due to leaked magnetic flux of permanent magnet, there occurs a thrust decrease (thrust ripple) in the vicinity of phase change position. Figure 13 shows the relationship between the table position and the thrust produced by a linear motor. Reference characters L1 - L4 each denotes a position where the phase is changed. If such a thrust characteristic can be detected beforehand, it is possible to determine a correction coefficient such as depicted in Figure 14. Actually, data for executing correction at a certain pitch P1 is memorized in a read only memory (ROM) 133. In accordance with the position of the table 101, a ripple correction controller 140 reads the correction coefficient data out of the memory 133, and a multiplier 130 multiplies it by a control signal outputted from the servo controller 108. In this example, the phase changing controller 109, the ripple correction controller 140 and the servo controller 108 are all accomplished by software computation by a central processing unit (microcomputer). The corrected signal outputted by the multiplier 130 is transmitted to the switch 111 through a digital-to-analog (D/A) converter 131. The correction pitch P1 is determined, within a range allowed by the capacity of the memory. The thrust ripple of the linear motor can be corrected in this manner. In the example of Figure 6 the coils (phases) are changed one by one at predetermined positions in response to digital signals (on/off signals). Thus, at the moment of changing the coil, the motor current decreases largely. Figure 8 shows details of a current feedback (FB) system of this example. For convenience of explanation of the operation, only coils 1 and 2 are illustrated. Denoted at 120 is a power operational amplifier of a gain 1000x for supplying an electric current to the motor. Denoted at 121 is an operational amplifier for detecting the magnitude of current on the basis of motor coil electric potential difference. Reference numerals 122 each denotes saturation of corresponding output. In this example, the output of the power operational amplifier 120 is saturated at ±28 V, while the operational amplifier 121 is saturated at ±12 V. Figures 9A - 9D illustrate waveforms when the coil to be energized at the moment 10 msec is changed from coil 1 to coil 2. In this example, the servo controller 108 supplies a signal to cause a flow of electric current of 3A. The phase changing controller 109 produces a digital signal (on/off signal) for turning corresponding switch 110 and, as a result, a step signal such as shown in Figure 9A or 9B is applied to corresponding switch. However, due to the presence of the saturation element 122 in the current loop, there is saturation in the output of the power operational amplifier 120 such as shown in Figure 9C. As a consequence of this, the current 1 and the current 2 do not follow the step signal, and the response such as illustrated in Figure 9A or 9B results. Since the sum of them provides the motor current, there occurs a large current decrease at the time of changing operation, such as shown in Figure 9D. Since the motor torque (thrust) is proportional to the magnitude of this electric current, the torque decreases in response to the changing operation. Now, some inconveniences to be caused if such torque decrease (phase change) occurs during constant speed movement or in the vicinity of a target position, will be explained. Figure 10 is a graph in a case where no torque decrease occurs during table movement, and Figure 11 a graph in a case where torque decrease (phase change) occurs. In the graph of Figure 11, as compared with that of Figure 10 having no torque decrease, the decrease of torque leads to degraded servo characteristic (followability), thus making the table position vibratory. In a semiconductor exposure apparatus and where exposure operation (scanning exposure operation) is to be made during constant speed movement, such uneven movement speed is undesirable since it leads to degraded circuit pattern printing precision. Further, any uneven torque in the vicinity of the target position causes decreased positioning precision or prolonged positioning time, thus adversely affecting the control system. In the example of Figure 12, a correction coefficient is calculated on the basis of thrust characteristic measured beforehand, and it is stored into the memory 133. The stored correction coefficient is read out in accordance with the position of the table 101, on the basis of which the thrust ripple of the linear motor is corrected. In this arrangement, however, if the thrust characteristic changes with time, it is no more possible to perform exact correction. In a critical case, the linear motor has to be demounted from the table and its thrust characteristic has to be measured again to renew the correction coefficient. This takes a lot of time. If on the other hand the correction pitch is made smaller to assure correction of better precision, a larger capacity of memory is required to store the correction coefficient. Now, a case where the thrust ripple correction is insufficient will be explained. Figure 15 shows a waveform in a case where no thrust ripple occurs during movement of a table. Figure 16 shows a case where thrust ripple occurs. Comparing the graph of Figure 16 with Figure 15 (no ripple), it is seen that the servo characteristic (followability) is degraded by the thrust ripple and the table position becomes vibratory. As described, in a semiconductor exposure apparatus and where exposure operation (scanning exposure operation) is to be made during constant speed movement, such uneven movement speed is undesirable since it degrades the circuit pattern printing precision. Further, uneven torque in the vicinity of the target position results in decreased positioning precision or prolonged positioning time, thus adversely affecting the control system. It is accordingly an object of the present invention to provide an improved positioning table device by which a decrease of torque in a multiphase motor at the time of phase change can be minimized to attain higher positioning precision and reduced positioning time. US-A-4390827 discloses a controller for a multiphase motor having a plurality of coils and adapted to be selectively energized to position an object, said controller comprising: position detecting means for detecting a position of said object with respect to a predetermined direction; feedback means for producing a current magnitude signal in accordance with an output of said position detecting means; coil selection means for selecting one or more of said coils to be energized by an electric current in accordance with the position of said object with respect to the predetermined direction; correcting means for correcting the current magnitude signal from said feedback means; and electric current supplying means for supplying an electric current to the coil or coils selected by said coil selection means in accordance with the corrected current magnitude signal. The present invention is characterised in that said coil selection means is adapted to produce a coil selection signal which changes quickly when a said coil is to be energized or de-energised, said correcting means includes, filter means for showing a change in said coil selection signal produced by said coil selection means, and multiplier means for multiplying the output of said filter means and said current magnitude signal to produce said corrected current magnitude signal. In one preferred form of the invention the coil selection signal comprises a digital signal to provide a delay to the coil selection signal. The delay time should preferably be determined so as not to cause saturation of the electric current control loop. With this arrangement, when the table is moved while the coils being changed sequentially in accordance with a predetermined energization pattern, a designated electric current to be applied to the servo circuit for coil energization can be changed slowly such that the combined electric current to the coils can be maintained substantially constant before and after the coil change. Thus, a decrease in torque at the coil change can be minimized. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a table positioning device according to a first embodiment of the present invention. Figure 2 is a diagram of electric current feedback means of a table positioning device according to the present invention. Figures 3A - 3D show waveforms at portions of the current feedback means. Figure 4 is a schematic view of a table positioning device according to a second embodiment of the present invention. Figure 5 is a perspective view of a rotary type stage device. Figure 6 is a schematic view of an example of table positioning device. Figure 7 is a schematic view of an example of linear motor coil energizing pattern memorized in a positioning table device. Figure 8 is a diagram of an example of electric current feedback means of a table positioning device. Figures 9A - 9D show waveforms at portions of the current feedback means. Figure 10 is a graph for explaining a case where no torque decrease occurs during table movement. Figure 11 is a graph for explaining a case where torque decrease (phase change) occurs during table movement. Figure 12 is a schematic view of an example of positioning table device. Figure 13 is a graph for explaining thrust characteristic of a multiphase linear motor. Figure 14 is a graph for explaining correction coefficient of an example of positioning table device. Figure 15 is a graph of speed waveform in a case where no thrust ripple occurs during table movement. Figure 16 is a graph of speed waveform in a case where thrust ripple occurs during table movement. Figure 1 is a schematic view of a positioning table device according to a first embodiment of the present invention. As illustrated, the table positioning device of this embodiment includes a linear motor coil means 4 having four coils (four phases) arrayed along a straight line. The motor coil means 4 is supported by a coil supporting member 3, and a pair of guides 2 are disposed in parallel to the array of the coils constituting the linear motor coil means 4. Mounted on the guides 2 are a pair of upper and lower top plates 1 which are provided to sandwich the linear motor coil 4 with the intervention of guide means 12. The upper and lower top plates 1 are slidably movable in the direction of the coil array, while being guided by the guides 2. The guide means 12 comprises any one of rolling, sliding and static pressure guiding means, for example, which can be selected suitably in consideration of the positioning precision or easiness of maintenance, for example. Each of the upper and lower top plates 1 is provided with movable magnet means 5 which comprises four permanent magnets disposed along the coil array with their magnetic poles disposed alternately. These movable magnets 5 are mounted to the table top plate 1 through the linear motor coil means 4 so that different magnetic poles are opposed to each other. Provided at the side of the guide 2 is a position detector 6 such as a linear scale, for example, for detecting the position of the table top plate 1. Connected to this position detector 6 is a counter unit 7 for detecting a position signal of the table. Connected to this counter unit 7 is a phase changing controller 9 for changing the coil phase. The controller 9 serves to produce coil selection signals (on/off signals) for the coils, for changing the coil to be energized, in accordance with the table position signal from the counter unit 7. The coil selection signal for each coil is applied to one of input terminals of a multiplier 19 corresponding to that coil. The coil phase is connected to the corresponding multiplier 19 through a corresponding current amplifier 11, such that in response to a coil selection signal (on/off signal) outputted from the controller 9 the output of the multiplier 19 is turned on or off, whereby voltage application to the coil is switched. Connected to the counter unit 7 is a servo controller 8 which serves to calculate a difference between the target position of the table and the current position thereof as detected through the counter unit 7, and to supply corresponding signals to the amplifiers 11 as designated values. Filter circuit 13 is connected to an output terminal of the controller 9, for supplying coil selection signals (digital outputs) for turning on or off the coils. This filter circuit 13 serves to provide filtering (delaying) to the coil selection signal. The coil selection signal filtered by the filter circuit 13 and the designated electric current magnitude for each coil supplied from the servo control circuit 8 are multiplied with each other in the multiplier 19, thus providing a designated electric current magnitude signal to be applied to the amplifier 11. In the device of Figure 1, by means of this filter circuit 13, the designated electric current magnitude signal, namely, the switching characteristic for each coil upon phase change can be controlled. The energization pattern of the coils may be such as illustrated in Figure 7. Figure 2 shows details of electric current feedback means of the present invention. For simplicity of explanation, only coil 1 and coil 2 are illustrated. In this example, as a practical and inexpensive filter circuit, a low pass filter comprising a resistor R and a capacitor C is used. Digital signal (on/off signal) passes this R-C low pass filter and then it is multiplied with the designated current magnitude from the servo controller 8. Thus, resultant designated electric current magnitude signal to be applied to the current amplifier takes a charge/discharge curve such as shown in Figure 3A or 3B, having slow delay time (time constant) t. With such slow designated value, the output of the power operational amplifier is not saturated (Figure 3C) and the electric current can follow the designated value. Adding the electric current curves of these coils at the time of phase change with each other results in the current magnitude such as shown in Figure 3D, as having a flat characteristic with the ripple being removed. Consequently, decrease of motor torque can be suppressed. Here, it is necessary to set the waveforms of designated current magnitude signals so that the combined current of the coils 1 and 2 do not substantially change. To this end, the delay time t should desirably be set so that the current loop is not saturated. Figure 4 is a schematic view of a positioning table device according to a second embodiment of the present invention. Since the device of this embodiment has a similar structure as that of the first embodiment, description will be made mainly to the difference thereof from the first embodiment. The positioning table device of the present invention may be provided, for the phase change, with a magnetic or optical position detecting element 14 separately from the position detector 6. For example, as shown in Figure 4, in the central portion of each coil constituting the linear motor coil means 4 of the device of the present invention, a Hall effect element 14 for detecting magnetic field is provided. Each Hall element 14 is connected to the phase change controller 9. As a magnetic field is applied to the effect element 14 by the permanent magnet 5, the Hall effect element detects the position of the magnetic pole of the permanent magnet 5 accurately and converts it into an electric signal which is then applied to the phase change controller 9. Then, the controller 9 operates to energize (turn on) the coil phases sequentially in accordance with the table position, essentially in the same manner as the first embodiment. The coil selection signals of digital outputs produced by the controller 9 for turning on/off the coils are communicated with the filter circuit 13. Each coil selection signal treated by this filter circuit and the designated current signal for each coil are multiplied with each other by a corresponding multiplier 19. Through this filter circuit, the switching characteristic of each coil can be controlled. Thus, also in this embodiment, decrease of torque can be minimized. In the first and second embodiments described above, an optical or magnetic linear scale is used as the position detector 6. However, if the invention is applied to a positioning table device in a semiconductor exposure apparatus, a laser distance measuring device having a higher resolution may preferably be used. Further, the present invention is applicable to a rotary type stage device having a multiphase motor of circular shape such as shown in Figure 5. In this example, movable magnet means 42 may be provided below a rotatable top plate 41. The motor current may be read in accordance with a position signal produced by a position detector 43, and the correction coefficient may be calculated to attain ripple correction. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: A controller for a multiphase motor (3,45) having a plurality of coils (4) and adapted to be selectively energized to position an object (1), said controller comprising: position detecting means (6) for detecting a position of said object (1) with respect to a predetermined direction; feedback means (8) for producing a current magnitude signal in accordance with an output of said position detecting means (6); coil selection means (9) for selecting one or more of said coils (4) to be energized by an electric current in accordance with the position of said object (1) with respect to the predetermined direction; correcting means (13,19) for correcting the current magnitude signal from said feedback means (8); and electric current supplying means (11) for supplying an electric current to the coil or coils selected by said coil selection means (9) in accordance with the corrected current magnitude signal; characterised in that said coil selection means (9) is adapted to produce a coil selection signal which changes quickly when a said coil is to be energized or de-energized, said correcting means (13,19) includes, filter means (13) for slowing a change in said coil selection signal produced by said coil selection means (9), and multiplier means (19) for multiplying the output of said filter means (13) and said current magnitude signal to produce said corrected current magnitude signal. A controller according to claim 1 wherein said coil selection means (9) is adapted to generate a digital coil selection signal and said filter means (13) is adapted to smooth the edges of the digital signal. A controller according to claim 1 or claim 2 wherein said coil selection means (9) is adapted to select one or more of said coils (4) to be energised in accordance with an output of said position detecting means (6). A controller according to claim 1 or claim 2 including second position detecting means (14) for detecting a position of said object (1) in said predetermined direction, wherein said coil selection means (9) is adapted to select one or more of said coils (4) to be energised in accordance with an output of said second position detecting means (6). A controller according to claim 1 wherein said second position detecting means (6) comprises a Hall effect element associated with each of said coils (4). A multiphase motor apparatus comprising the controller according to any preceding claim, and a multiphase motor having a plurality of coils adapted to be selectively energized to position an object with respect to a predetermined direction. A positionable table arrangement comprising a table (1) having a permanent magnet (5) provided thereon; guide means (2,12) for guiding movement of said table (1); and a multiphase motor apparatus according to claim 6, wherein said coils (4) of said multiphase motor are opposed to said permanent magnet.

Inventors: OHISHI SHINJI, OHISHI, SHINJI, OHISHI, SHINJI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: H02K41
IPC codes: H02K41, G03F7, G05B19